1. Field of Invention
The present invention relates to high-speed data communications cables. More particularly, it relates to cables including shaped separators and jackets.
2. Discussion of Related Art
High-speed data communications media include pairs of wire twisted together to form a balanced transmission line. Such pairs of wire are referred to as twisted pairs. One common type of conventional cable for high-speed data communications includes multiple twisted pairs that may be bundled and twisted (cabled) together then covered with a jacket to form the cable.
Modern communication cables must meet electrical performance characteristics required for transmission at high frequencies. When twisted pairs are closely placed, as may be the case in a multi-pair cable, electrical energy may be transferred from one twisted pair to another. Such energy transferred between pairs is referred to as crosstalk and is generally undesirable. Crosstalk causes interference to the information being transmitted through the twisted pair(s) and can reduce the data transmission rate and cause an increase in the bit error rate. The Telecommunications Industry Association and the Electronics Industry Association (TIA/EIA) have developed standards which specify specific categories of performance for cable impedance, attenuation, skew and crosstalk isolation. The International Electrotechnical Commission (IEC) has also defined standards for data communication cable crosstalk, including ISO/IEC 11801. One high-performance standard for 100Ω cable is ISO/IEC 11801, Category 5; another is ISO/IEC 11801 Category 6.
In twisted pairs, the rate of twist is defined as a specified distance between twists along the longitudinal direction, that distance being referred to as the pair lay or twist lay. When adjacent twisted pairs have the same pair lay and/or twist direction, they tend to lie within a cable more closely spaced than when they have different pair lays and/or twist direction. Such close spacing may increase the amount of undesirable crosstalk which occurs between adjacent pairs. Therefore, twisted pairs within a cable are sometimes given unique pair lays so as to reduce the crosstalk between twisted pairs of a cable. Twist direction may also be varied. Along with varying pair lays and twist directions, individual solid metal or woven metal pair shields are sometimes used to electromagnetically isolate pairs from one another.
In some cables, a separator is used to separate one twisted pair from another to improve crosstalk between the pairs and/or to provide added structural stability to the cable. For example, referring to FIG. 1, there is illustrated an example of a cable 100 including a plurality of twisted pairs 103 and a conventional separator 200. The twisted pairs 103 are spaced about the separator 200 which provides physical separation among the pairs. The separator can also provide structural stability to the cable. Generally, the separator 200 comprises a solid, round rod, as illustrated, and may be made of a suitable dielectric material. The cable may be finished with a jacket 202 provided around the twisted pairs 103 and the separator 200.
In building design, many precautions are taken to resist the spread of flame and the generation of and spread of smoke throughout a building in case of an outbreak of fire. Clearly, it is desired to protect against loss of life and also to minimize the costs of a fire due to the destruction of electrical and other equipment. Therefore, wires and cables for in building installations are required to comply with the various flammability requirements of the National Electrical Code (NEC) and/or the Canadian Electrical Code (CEC).
Cables intended for installation in the air handling spaces (i.e. plenums, ducts, etc.) of buildings are specifically required by NEC or CEC to pass the flame test specified by Underwriters Laboratories Inc. (UL), UL-910, or its Canadian Standards Association (CSA) equivalent, the FT6. The UL-910 and the FT6 represent the top of the fire rating hierarchy established by the NEC and CEC respectively. Cables possessing this rating, generically known as “plenum” or “plenum rated”, may be substituted for cables having a lower rating (i.e. CMR, CM, CMX, FT4, FT1 or their equivalents), while lower rated cables may not be used where plenum rated cable is required. Cables conforming to NEC or CEC requirements are characterized as possessing superior resistance to ignitability, greater resistant to contribute to flame spread and generate lower levels of smoke during fires than cables having a lower fire rating. Conventional designs of data grade telecommunications cables for installation in plenum chambers have a low smoke generating jacket material, e.g. of a PVC formulation or a fluoropolymer material, surrounding a core of twisted conductor pairs, each conductor individually insulated with a fluorinated ethylene propylene (FEP) insulation layer. Cable produced as described above satisfies recognized plenum test requirements such as the “peak smoke” and “average smoke” requirements of the Underwriters Laboratories, Inc., UL910 Steiner test and/or Canadian Standards Association CSA-FT6 (Plenum Flame Test) while also achieving desired electrical performance in accordance with EIA/TIA-568A for high frequency signal transmission.